Switching power converters, such as switching DC-to-DC converters, are known. Switching power converters typically have a higher efficiency and a smaller size than linear power converters with corresponding power ratings. Accordingly, switching power converters are widely used in applications requiring small size and/or high efficiency, such as in battery powered portable electronic devices.
Many switching power converters require one or more inductors for temporary storage of energy during each converter switching cycle. One example of a switching power converter requiring an inductor is a buck DC-to-DC converter, which requires at least one inductor. Switching power converter inductors typically handle large magnitude, high frequency alternating currents. Accordingly, significant power is lost in the inductors due to factors including winding resistive power losses, which increase with the square of winding current, and core losses, which increase with increasing switching converter operating frequency. Inductor power loss is undesirable, particularly in battery powered portable applications, where it is desirable to conserve battery power and minimize use of cooling components such as heat sinks and/or fans to remove heat resulting from the power loss.
One known inductor commonly employed in switching converters includes a single-turn “staple” winding wound through a ferrite magnetic core. This single-turn inductor advantageously has a relatively low cost and winding resistance. The ferrite material also exhibits low core losses at high operating frequencies relative to other core materials, such as powdered iron. However, this single-turn inductor may not be suitable for applications requiring small inductor size, particularly when a large inductance value and/or high efficiency are required. Inductance of such an inductor is directly proportional to core cross sectional area, and core losses are indirectly proportional to core cross sectional area. Accordingly, for a given core material and winding configuration, core cross sectional area may be increased to increase inductance and/or decrease core losses. But, increasing core cross sectional area correspondingly increases physical inductor size (e.g., height). Large inductors are undesirable or unacceptable in many applications, such as in space constrained portable device applications. Increasing core cross sectional area also generally increases inductor cost.
A single-turn inductor typically has an air-gap in its core, and inductance can also be increased by decreasing thickness of such gap. However, decreasing the gap's thickness correspondingly increases core magnetic flux density, which typically increases core losses. Core losses generally increase as flux density increases, such as in proportion to the square or even the cube of flux density. Accordingly, core losses may rapidly increase as the gap's thickness is decreased. Additionally, small gap thickness results in the inductor saturating at relatively low currents, thereby limiting the inductor's maximum operating current.
Some drawbacks of a single-turn inductor can be overcome by increasing the number of turns to two or more. Inductance is proportional to the square of number of turns. Additionally, increasing the number of turns allows for a core's air gap thickness to be increased while maintaining the same inductance value, thereby lowering magnetic flux density and associated core losses. Accordingly, increasing the number of turns can increase inductance or decreases core losses without increasing core cross sectional area. However, present multi-turn inductors typically suffer from problems such as being difficult and costly to manufacture and/or having a high winding resistance.
There have been attempts to produce low cost multi-turn inductors. For example, FIG. 1 shows a perspective view of one prior art surface mount inductor 100 including a rectangular magnetic core 102 and two single-turn staple windings 104, 106 wound through magnetic core 102. Only the outline of core 102 is shown in FIG. 1 so that windings 104, 106 are visible. Inductor 100 can be configured as a two-turn inductor by electrically connecting together windings 104, 106 in series. For example, solder tabs 108, 110 may be electrically connected together in series using a printed circuit board (“PCB”) trace underlying the inductor such that windings 104, 106 are electrically connected in series and solder tabs 112, 114 provide an electrical interface to each end of the series connected windings. Each winding 104, 106 has a DC resistance of 0.52 milliohm, for example. Single-turn staple windings are typically inexpensive to manufacture, and inductor 100 is therefore typically inexpensive to manufacture, even though inductor 100 can be configured as a two-turn inductor. However, the configuration of inductor 100 results in high resistive power losses in typical applications.
For example, FIG. 2 is a top plan view of one printed circuit board footprint 200 for use with inductor 100 in a two-turn configuration. Footprint 200 includes pads 202, 204, 206, 208 for respectively connecting to solder tabs 108, 110, 112, 114 of windings 104, 106. Pads 202, 204 are electrically connected together via a PCB trace 210. PCB trace 210 is typically a thin copper foil having a relatively high resistance. For example, PCB traces in portable, high density applications are commonly formed of “1 ounce” copper foil, which has thickness of about 35 microns. In one representative configuration, PCB trace 210 has a resistance of approximately 0.6 milliohm between pads 202, 204, which is larger than the DC resistance of each winding 104, 106. Accordingly, although inductor 100 can be configured as a two-turn surface mount inductor, significant power will be lost due to high resistance of PCB trace 210 required to connect windings 104, 106 in series.